The family of receptors that transmit signals through the activation of heterotrimeric GTP binding proteins (G proteins) constitutes the largest group of cell surface proteins involved in signal transduction. These receptors participate in a broad range of important biological functions and are implicated in a number of disease states. More than half of all drugs currently available influence GPCRs. These receptors affect the generation of small molecules that act as intracellular mediators or second messengers, and can regulate a highly interconnected network of biochemical routes controlling the activity of several members of the mitogen-activated protein kinase (MAPK) superfamily.
In fact, the activation of members of the mitogen-activated protein kinase (MAPK) family represents one of one of the major mechanisms used by eukaryotic cells to transduce extracellular signals into cellular responses (J. Blenis, Proc. Natl. Acad. Sci., USA 90:5889 (1993) (1); Blumer et al., TIBS 19:236 (1994) (2); Cano et al., TIBS 20:117 (1995) (3); Seger et al., FASEB J. 9:726 (1995) (4); R. J. Davis, TIBS 19:470 (1994) (5)). The MAPK superfamily consists of the p42 (ERK2)/p44 (ERK1) MAPKs and the stress-activated protein kinases, c-Jun N-terminal kinase (JNK) and p38 MAPK. (Robinson and Dickenson, Eur. J. Pharmacol. 413(2-3):151-61 (2001)(6)).
Mitogen-activated protein kinase (MAPKs) (also called extracellular signal-regulated kinases or ERKs) are rapidly activated in response to ligand binding by both growth factor receptors that function as tyrosine kinases (such as the epidermal growth factor (EGF) receptor) and receptors that are complexed with heterodimeric guanine nucleotide binding proteins (G proteins) such as the thrombin receptor. In addition, receptors like the T cell receptor (TCR) and B cell receptor (BCR) are non-covalently associated with src family tyrosine kinases which activate MAPK pathways. Specific cytokines like tumor necrosis factor (TNFalpha) can also regulate MAPK pathways. The MAPKs appear to integrate multiple intracellular signals transmitted by various second messengers. MAPKs phosphorylate and regulate the activity of enzymes and transcription factors including the EGF receptor, Rsk 90, phospholipase A2, c-Myc, c-Jun and E1K-1/TCF. Although the rapid activation of MAPKs by tyrosine kinase receptors is dependent on Ras, G protein-mediated activation of MAPK also occurs through pathways dependent and independent of Ras.
Particularly, it is known that the activation of MAP/ERK kinase which is induced by GPCRs involves both the G alpha and G beta gamma subunits and further involves a common signaling pathway with receptor-tyrosine-kinases. (Lopez-Ilasaca, Biochem. Pharmacol. 56(3): 269-77 (1998) (7)). For example, the G protein beta gamma subunit has been shown to activate Ras, Raf and MAP kinase in HEK293 cells. (Ito et al., FEBS Lett. 368(1): 183-7 (1995) (8)).
Additionally of relevance to the present invention, within the last several years, a number of groups including the present assignee Senomyx Inc., have reported the identification and cloning of genes from two GPCR families that are involved in taste modulation and have obtained experimental results that provide a greater understanding of taste biology. These results indicate that bitter, sweet and amino acid taste, also referred as umami taste, is triggered by activation of two types of specific receptors located at the surface of taste receptor cells (TRCs) on the tongue i.e., T2Rs and T1Rs (9-11) (Gilbertson et al., Corr. Opin. Neurobiol., 10(4):519-27 (2000); Margolskee, R F, J. Biol. Chem. 277(1):1-4 (2002); Montmayeur et al., Curr. Opin. Neurobiol., 12(4):366-71 (2002)). It is currently believed that at least 26 and 33 genes encode functional receptors (T2Rs) for bitter tasting substances in human and rodent respectively (11-13) (Montmayour et al., Curr. Opin. Neurobiol., 12(4):366-71 (2002); Adler et al., Cell 100(6):693-702 (2000); Matsunami et al., Nature 404(6678):601-4 (2000)). By contrast there are only 3 T1Rs, T1R1, T1R2 and T1R3, which are involved in umami and sweet taste (14-16) (Li et al., Proc. Natl Acad Sci., USA 99(7):4692-6 (2002); Nelson et al., Nature (6877):199-202 (2002); Nelson et al., Cell 106(3):381-96 (2001)). Structurally, the T1R and T2R receptors possess the hallmark of G protein-coupled receptors (GPCRs), i.e., 7 transmembrane domains flanked by small extracellular and intracellular amino- and carboxyl-termini respectively.
T2Rs which have been cloned from different mammals including rats, mice and humans (12) (Adler et al., Cell 100(6): 611-8 (2000)). T2Rs comprise a novel family of human and rodent G protein-coupled receptors that are expressed in subsets of taste receptor cells of the tongue and palate epithelia. These taste receptors are organized in clusters in taste cells and are genetically linked to loci that influence bitter taste. The fact that T2Rs modulate bitter taste has been demonstrated in cell-based assays. For example, mT2R-5, hT2R-4 and mT2R-8 have been shown to be activated by bitter molecules in in vitro gustducin assays, providing experimental proof that T2Rs function as bitter taste receptors. (80) (Chandrasheker et al., Cell 100(6): 703 (2000)).
The present assignee has filed a number of patent applications relating to various T2R genes and the corresponding polypeptides and their use in assays, preferably high throughput cell-based assays for identifying compounds that modulate the activity of T2Rs. These Senomyx applications i.e., U.S. Ser. No. 09/825,882, filed on Apr. 5, 2001, U.S. Ser. No. 191,058 filed Jul. 10, 2002 and U.S. Provisional Application Ser. No. 60/398,727, filed on Jul. 29, 2002 all incorporated by reference in their entireties herein. Additionally, the present assignee has exclusively licensed patent applications relating to T2R genes which were filed by the University of California i.e., U.S. Ser. No. 09/393,634, filed on Sep. 10, 1999 (recently allowed) and U.S. Ser. No. 09/510,332, filed Feb. 22, 2000, that describe various mouse, rat and human T2R sequences and the use thereof in assays for identifying molecules that modulate specific T2Rs and which modulate (enhance or block) bitter taste. These applications and the sequences contained therein are also incorporated by reference in their entireties herein.
Further, the present assignee and its exclusive licensor, the University of California, have filed a number of patent applications relating to human and rodent T1R taste receptors. Specifically, Senomyx has filed patent applications Ser. No. 09/897,427, filed on Jul. 3, 2001, U.S. Ser. No. 10/179,373, filed on Jun. 26, 2002, and U.S. Ser. No. 09/799,629, filed on Mar. 7, 2001, all of which and the sequences contained therein are incorporated by reference in their entirety herein. Additionally, the University of California has filed a number of applications exclusively licensed by Senomyx including U.S. Ser. No. 09/361,631, filed Jul. 27, 1999, now U.S. Pat. No. 6,383,778, issued on May 7, 2002 and U.S. Ser. No. 09/361,652, filed on Jul. 27, 1999, which relates to cloned rat, mouse and human T1R1 and T1R2 genes and the use of the genes and corresponding polypeptides to identify T1R modulators. These University of California applications and the sequences contained therein are also incorporated by reference in their entirety herein.
The three T1R gene members T1R1, T1R2 and T1R3 form functional heterodimers that specifically recognize sweeteners and amino acids (14-16) (Li et al., Proc. Natl Acad Sci., USA 99(7):4692-6 (2002); Nelson et al., Nature (6877):199-202 (2002); Nelson et al., Cell 106(3):381-96 (2001)). Functional studies performed in HEK293 cells expressing the promiscuous G protein Gα15/16, also disclosed therein have shown that the rodent and human T1R2/T1R3 combination recognizes natural and artificial sweeteners (14-16) (Li et al., Proc. Natl Acad Sci., USA 99(7):4692-6 (2002); Nelson et al., Nature (6877):199-202 (2002); Nelson et al., Cell 106(3):381-96 (2001)) while the rodent and human T1R1/T1R3 combination recognizes several L-amino acids and monosodium glutamate (MSG), respectively (14, 15) (Li et al., Proc. Natl Acad Sci., USA 99(7):4692-6 (2002); Nelson et al., Nature (6877):199-202 (2002)). These results, demonstrate that T1Rs are involved in sweet and umami taste.
Particularly, the co-expression of T1R1 and T1R3 in recombinant host cells results in a hetero-oligomeric taste receptor that responds to umami taste stimuli. Umami taste stimuli include by way of example monosodium glutamate and other molecules that elicit a “savory” taste sensation. By contrast, the co-expression of T1R2 and T1R3 in recombinant host cells results in a hetero-oligomeric sweet taste receptor that responds to both naturally occurring and artificial sweeteners. As with T2Rs, T1R DNAs and the corresponding polypeptides have significant application in cell and other assays, preferably high throughput assays, for identifying molecules that modulate T1R taste receptors; particularly the T1R2/T1R3 receptor (sweet receptor) and the T1R1/T1R3 receptor (umami receptor). T1R modulators can be used as flavor-affecting additives in foods, beverages and medicines.
The patents and patent application referenced above, which are incorporated by reference in their entirety herein, disclose a number of assay methods, including cell-based high throughput screening assays for identifying T1R and T2R agonists and antagonists. However, notwithstanding what is disclosed therein, novel and improved assays for identifying T1R and T2R agonists and antagonists are still needed. In particular other high throughput assays that provide for the rapid and accurate identification of T1R or T2R agonists and antagonists would be beneficial. Also, a greater understanding of what conditions and materials yield functional T1Rs and T2Rs and assays based on this greater understanding would further be beneficial.